Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227, which is incorporated herein by reference in its entirety.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 30 formed of titanium for example. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102 and 104 are shown), such that the electrodes 106 form an electrode array 110. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 102, labeled E1-E8, and eight electrodes on lead 104, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 102 and 104 couple to the IPG 100 using lead connectors 38a and 38b, which are fixed in a header material 36.
As shown in FIG. 2, the IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors mounted to the PCB 16. Two coils are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller 12; and a charging coil 18 for charging or recharging the IPG's battery 26 using an external charger 50. The telemetry coil 13 can be mounted within the header 36 of the IPG 100 as shown.
As just noted, an external controller 12, such as a hand-held programmer or a clinician's programmer, is used to send data to and receive data from the IPG 100. For example, the external controller 12 can send programming data to the IPG 100 to dictate the therapy the IPG 100 will provide to the patient. Also, the external controller 12 can act as a receiver of data from the IPG 100, such as various data reporting on the IPG's status. The external controller 12, like the IPG 100, also contains a PCB 70 on which electronic components 72 are placed to control operation of the external controller 12. A user interface 74 similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the external controller 12.
Wireless data transfer between the IPG 100 and the external controller 12 takes place via inductive coupling. To implement such functionality, both the IPG 100 and the external controller 12 have coils 13 and 17 respectively. Either coil can act as the transmitter or the receiver, thus allowing for two-way communication between the two devices. When data is to be sent from the external controller 12 to the IPG 100 for example, coil 17 is energized with alternating current (AC), which generates a magnetic field 29, which in turn induces a voltage in the IPG's telemetry coil 13. The generated magnetic field 29 is typically modulated using a communication protocol, such as a Frequency Shift Keying (FSK) protocol, which is well known in the art. The power used to energize the coil 17 can come from a battery 76, which like the IPG's battery 26 is preferably rechargeable, but power may also come from plugging the external controller 12 into a wall outlet plug (not shown), etc. The induced voltage in coil 13 can then be demodulated at the IPG 100 back into the telemetered data signals. To improve the magnetic flux density, and hence the efficiency of the data transfer, the IPG's telemetry coil 13 may be wrapped around a ferrite core 13′.
The external charger 50 is used to charge (or recharge) the IPG's battery 26. Specifically, and similarly to the external controller, the coil 17′ is energized with an AC current to create a magnetic field 29. This magnetic field 29 induces a current in the charging coil 18 within the IPG 100, which current is rectified to DC levels, and used to recharge the battery 26. The external charger 50 will generally have many of the same components as the external controller 12, and therefore have similar element numerals, denoted with prime symbols. However, while sufficient for purposes of this disclosure to view the external controller 12 and charger 50 as essentially the same, one skilled in the art will realize that external controllers 12 and chargers 50 will have pertinent differences as dictated by their respective functions.
As is well known, inductive transmission of data or power can occur transcutaneously, i.e., through the patient's tissue 25, making it particularly useful in a medical implantable device system. During the transmission of data or power, the coils 13 and 17, or 18 and 17′, preferably lie along a common axis in planes that are parallel. Such an orientation between the coils will generally improve the coupling between them, but deviation from ideal orientations can still result in suitably reliable data or power transfer.
A rechargeable battery 26 for the illustrated IPG 100 is shown in FIGS. 3A and 3B, which depicts an exemplary 3.6V, 200 mAh lithium ion battery, part number QL02001-A, manufactured by Quallion, LLC. As shown, the battery 26 is generally in the shape of a “squashed” cylinder. The battery 26 includes an external housing 150; its internal components can be seen in cross section in FIG. 3B. Specifically noticeable are four plates that have been spirally wound within the housing 150: a conductive anode plate 140, a conductive cathode plate 142, and two separator plates 144. An electrolyte (not shown) fills the spaces between the various plates, as is well known.
FIG. 4 shows the anode plate 140, the cathode plate 142, and the separator plates 144 unrolled as separate sheets for simplicity. The anode plate 140 includes anodic materials such as graphite for example. The anode plate 140 can also include a substrate upon which the anodic material is placed, such as a titanium substrate. However, the delineation of such layers in the anode plate 140 is not shown in FIG. 4 for simplicity. The cathode plate 142 includes Lithium-based cathodic materials such as LiC6, Li4Ti5O12, LiCoO2, or LiNiCoO2 for example. Again, the cathode plate 142 can include a substrate upon which the cathodic materials are placed, such as aluminum. The separator plates 144 can comprise many materials, such as polyethylene. Other details concerning lithium ion batteries and their materials can be found at U.S. Pat. No. 6,553,263 and also at http://en.wikipedia.org/wiki/Lithium_ion_battery, a copy of which is included in the Information Disclosure Statement filed herewith, both of which are incorporated herein by reference in their entireties.
As is typical, the anode plate 140 and cathode plate 142 are coupled to two external battery terminals 132 and 130, respectively, as shown in FIGS. 3A and 4. The cathode battery terminal 130 is coupled to the battery case 150, while the anode battery terminal 132 is insulated from the battery case by an insulator 133 (FIG. 3A). Whether the battery housing 150 is coupled to the anode or cathode depends on the design, and can vary. The battery terminals 130 and 132 are ultimately electrically connected to the PCB 16 (FIG. 2) in the IPG 100, such as by soldering.
The inventor has noticed that an implantable medical device having a rechargeable lithium ion battery such as the battery 26 may interfere with a patient's use of magnetic resonance imaging (MRI). In particular, the inventor is concerned that when an MRI operation is being performed on a patient with an implantable device such as IPG 100, the MRI's magnetic fields may interfere with the battery 26. As a result of such interference, the battery 26 may become excessively hot and may even vibrate.
FIG. 5 shows a cross section of an MRI machine 500 with a patient 545 inside. The MRI machine 500 includes a static magnet 502 and three gradient magnets 504, 506 and 508. The three gradient magnets 504, 506 and 508 are each aligned in X, Y and Z planes respectively. Gradient magnets 504, 506 and 508 generate an alternating X magnetic field 520, an alternating Y magnetic field 530, and an alternating Z magnetic field 540 respectively. The static magnet 502 generates a static magnetic field 550 of 0.5-tesla to 3.0-tesla or more, or 5,000 to 30,000 gauss or more, which is a very strong magnetic field.
When a patient having an IPG 100 with a battery 26 is placed inside the MRI 500 machine, the alternating magnetic fields 520, 530, and 540 of the MRI induce circulating currents in the conductive plates 140 and 142 and the housing 150 of the battery 26. These currents are known as eddy currents, and are shown generically in FIG. 6 as a current 606 responsive to one of the alternating magnetic fields 602 (e.g., 520, 530, or 540) of the machine 500. (Eddy currents 606 are only shown in a major surface of the battery 26 in FIG. 6 for convenience, but it will be understood that the plurality of alternating fields 520, 530, and 540 would cause other eddy currents in other surfaces of the battery 26). Such eddy currents 606 can heat the battery 26, which can in turn heat the IPG 100. Such heat can cause discomfort for the patient and can also result in decreased battery life.
The inventor considers the eddy currents 606 in the battery 26 problematic in another way which relates to the MRI environment. Pursuant to a principle known as Lenz's law, eddy currents 606 induce alternating magnetic field 604 which generally opposes the field 602 that created it. This induced alternating magnetic field 604 can react with the strong static magnetic field 550 in the MRI machine 500. Generally speaking, the static magnetic field 550 will have fixed north and south poles, while the induced alternating magnetic field 604 will have north and south poles that alternate. These two fields 604 and 550 will react with one another so as to place a mechanical force on the battery 26 that alternatively pushes and pulls the battery 26 along an axis with a frequency matching that of the alternating field 604. In other words, the induced alternating magnetic field 604 will cause the battery 26 to vibrate in the presence of the static magnetic field 550. The larger the magnitude of the eddy currents 606, the stronger the induced alternating magnetic field 604, and hence the stronger the vibrational force on the battery 26. Such vibrations can weaken mechanical fastenings of the battery 26 inside of the housing 30 of the IPG 100, and may result in breaking solder joints or damaging other components in the IPG 100.
Additionally, the induced alternating magnetic fields 604 generated by the eddy currents 606 in the battery 26 can cause distortions in the area around the IPG, thus negatively affecting MRI image quality.
Given the shortcomings of the rechargeable battery of the prior art, it would be beneficial to provide an improved rechargeable battery for an implantable medical device, and this disclosure presents such a solution.